It still doesn't solve the problem of people staring at the "join game" screen until a spot opens up in one of the games. Doesnt matter what level they are, I would consider this leeching -- leeching off someone else's work to get into the cave. If the devs really want people to work to get in there, they should do away with the caves showing up on the "join game" screen. Any open slots should be filled in by invited friends.
Edit: at the very least, this might take a bit more planning, making sure you have a few friends available before starting the game.
Also, something would of course have to be done to prevent people from getting locked out of the game if the get disconnected. Fixing the host problem would be one way to deal with that -- so they know their friends can hold the spot for them if they get lagged out.